A MRAM (magnetic random access memory) is regarded as a nonvolatile memory which is favorable from a viewpoint of high integration and high-speed operation (e.g. refer to Japanese Laid-Open Patent Applications JP-P 2003-272375A, JP-P 2002-140889A, JP-P 2004-5972A, JP-P 2003-346475A and so on). In the MRAM, magnetic resistance elements are utilized which exhibit a “magnetoresistance effect” such as a TMR (tunnel magnetoresistance) effect. For example, an MTJ (magnetic tunnel junction) in which a tunnel barrier layer is held between two-layer ferromagnetic layers is formed in the magnetoresistance elements. The two-layer ferromagnetic layers are made of a pinned layer whose magnetization orientation is fixed and a free layer whose magnetization orientation can be switched.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization orientation of the free layer is “antiparallel” to that of the pinned layer is larger than a resistance value (R) of the MTJ when the magnetization orientation of the free layer is “parallel” to that of the pinned layer due to the magnetoresistance effect. The MRAM uses the magnetoresistance elements having the MTJ as memory cells, and stores data by using variations of a resistance value of the magnetoresistance element in a nonvolatile manner. Data is written into the memory cell by switching the magnetization orientation of the free layer.
An asteroid method and a toggle method have been known as a method to write data with respect to the MRAM. According to these write methods, a switching magnetic field required to switch magnetization of the free layer increases in substantially inverse proportion to a size of the memory cell. That is, a write current tends to increase based on miniaturization of the memory cell.
A write method is proposed, which can suppress an increase in the write current accompanied by the miniaturization. The method is a spin transfer method (e.g. refer to Grollier et al, “Spin-polarized current induced switching in Co/Cu/Co pillars”, Applied Physics Letters, Vol. 78, pp. 3663, 2001; Yagami and Suzuki, “Research Trends in Spin Transfer Magnetization Switching”, Journal of magnetic society of Japan, Vol. 28, No. 9, 2004). According to the spin transfer method, a spin-polarized current is transferred to a ferromagnetic conductor, so that magnetization is switched by a direct interaction between spin of conductive electrons which carry the current and magnetic moment of the conductor (referred to as a “spin transfer magnetization switching” hereinafter). An outline of the spin transfer magnetization switching will be explained referring to FIG. 1.
In FIG. 1, a magnetoresistance element 1 is provided with a free layer 2 and a pinned layer 4 as magnetic material layers, and a tunnel barrier layer 3 as a nonmagnetic material layer which is held between the free layer 2 and the pinned layer 4. Here, the pinned layer 4 whose magnetization orientation is fixed is formed to be thicker than the free layer 2, serving as a spin filter, i.e. a mechanism to create a spin-polarized current. A state of having a parallel magnetization orientation between the free layer 2 and the pinned layer 4 is made to correspond to data “0”, while a state of having an antiparallel magnetization orientation therebetween is made to correspond to data “1”.
The spin transfer magnetization switching as shown in FIG. 1 is realized by a CPP (current perpendicular to plane) method, where a write current IW is vertically transferred to a film plane. More specifically, in transition from the data “0” to data “1”, a write current IW is made to flow from the pinned layer 4 to the free layer 2. In this case, electrons having the same spin state as that of electrons in the pinned layer 4 as the spin filter move from the free layer 2 to the pinned layer 4. Magnetization of the free layer 2 is then switched by spin transfer effects brought by supplying and receiving spin angular momentum. In transition from the data “1” to data “0”, the write current IW is made to flow from the free layer 2 to the pinned layer 4. In this case, electrons having the same spin state as that of electrons in the pinned layer 4 as the spin filter move from the pinned layer 4 to the free layer 2. The magnetization of the free layer 2 is switched by the spin transfer effects.
Data is thus written by movement of spin electrons in the spin transfer magnetization switching. A magnetization orientation of the free layer 2 can be defined by a direction of a spin-polarized current which is vertically transferred to the film plane. A write (i.e. magnetization switching) threshold value here is known as being dependent on current density. Accordingly, a write current which is required to switch magnetization decreases based on a size decrease of a memory cell. Since miniaturization of a memory cell is accompanied by a decrease in the write current, the spin transfer magnetization switching is important to realize a MRAM having a large capacity.
It is necessary for any write method to change a magnetization state of the free layer in a write operation of the MRAM. Therefore, there is a probability that desired data is not written into a memory cell (referred to as a “probability of erroneous writing” hereinafter). An increase in the probability of erroneous writing is caused by a decreased write current to suppress power consumption and a shortened write time to realize high speed operation.
Japanese Laid-Open Patent Application JP-P 2003-115577A discloses a recording/reproducing method of a nonvolatile magnetic thin film memory device aimed to suppress write failures. According to the recording/reproducing method, test writing is carried out in a memory cell used for test writing before recording data. After confirming a record of the test writing, actual data is written. In this case, although an amount of time to write data increases, a probability of performing a normal write operation increases even in different temperature environment circumstances.